Physical protocol data unit transmission method employed by wireless fidelity multi-link device

ABSTRACT

A physical protocol data unit (PPDU) transmission method includes aligning parameters of a plurality of links for constraining the plurality of links to have same capacity for PPDU transmission, and transmitting PPDUs via the plurality of links. Highest capacity supported by one of the plurality of links is different from highest capacity supported by another of the plurality of links. Each of the PPDUs is generated and transmitted under the same parameters. Ending time instants of transmission of the PPDUs are aligned.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application No.62/909,831, filed on Oct. 3, 2019 and incorporated herein by reference.

BACKGROUND

The present invention relates to wireless communications, and moreparticularly, to a physical protocol data unit (PPDU) transmissionmethod employed by a wireless fidelity (WiFi) multi-link device (MLD).

In a WiFi multi-link operation (MLO), there exist several links betweentwo MLDs, including one access point (AP) and one non-AP station (STA),that occupy different radio-frequency (RF) bands. These links canoperate independently to increase the overall throughput and/or toimprove the connection stability. Regarding a synchronous mode of MLO,an MLD may synchronize its transmit (TX) timing and/or receive (RX)timing of multiple links. However, each link has its own capacity thatis based on several parameters, including bandwidth (BW), number ofspatial streams (NSS), modulation and coding mechanism (MCS), etc.Capacities of links can be very different. Hence, for the same upperlayer content to be transmitted over these links, physical protocol dataunit (PPDU) lengths may be very different, thus resulting in PPDU endingtime misalignment.

The PPDU ending time misalignment may occur in some scenarios. In afirst scenario, duplicate transmission is performed over a high capacitylink and a low capacity link. Due to link capacity difference, the samecontents transmitted over different links result in very different PPDUlengths. In a second scenario, a low capacity link's PPDU goes first anda high capacity link's PPDU goes later. Due to link capacity differenceand transmission timing difference, the data left on the high capacitylink is not enough to fulfill the PPDU length requirement to besynchronized with the low capacity link's PPDU. In a third scenario, thePPDU takes 80% of data on the first link, and the 20% rest datatransmitted later on the second link results in a much shorter PPDU dueto the fact that uncertain second link transmission makes thetransmitter to load more data to the early coming transmissionopportunity.

Thus, there is a need for an innovative PPDU transmission design whichcan deal with any of the above-mentioned scenarios to help a WiFi MLD toachieve PPDU ending time alignment of multiple links.

SUMMARY

One of the objectives of the claimed invention is to provide a physicalprotocol data unit (PPDU) transmission method employed by a wirelessfidelity (WiFi) multi-link device (MLD). For example, the proposed PPDUtransmission method can achieve PPDU ending time alignment of multiplelinks under any of the above-mentioned scenarios.

According to a first aspect of the present invention, an exemplaryphysical protocol data unit (PPDU) transmission method is disclosed. Theexemplary PPDU transmission method includes: aligning, by a wirelessfidelity (WiFi) multi-link device (MLD), parameters of a plurality oflinks for constraining the plurality of links to have same capacity forPPDU transmission, wherein highest capacity supported by one of theplurality of links is different from highest capacity supported byanother of the plurality of links; and transmitting PPDUs via theplurality of links, wherein each of the PPDUs is generated andtransmitted under the same parameters, and ending time instants oftransmission of the PPDUs are aligned

According to a second aspect of the present invention, an exemplaryphysical protocol data unit (PPDU) transmission method is disclosed. Theexemplary PPDU transmission method includes: setting parameters of eachof a plurality of links for enabling the plurality of links to havedifferent capacity for PPDU transmission, wherein parameters of one ofthe plurality of links are different from parameters of another of theplurality of links, and highest capacity supported by said one of theplurality of links is higher than highest capacity supported by saidanother of the plurality of links; setting, by a wireless fidelity(WiFi) multi-link device (MLD), a content carried by a first PPDUtransmitted via said one of the plurality of links to ensure that anending time instant of transmission of the first PPDU transmitted viasaid one of the plurality of links and an ending time instant oftransmission of a second PPDU transmitted via said another of theplurality of links are aligned; and transmitting PPDUs via the pluralityof links, wherein one PPDU is transmitted via each of the plurality oflinks, and the PPDUs comprise the first PPDU and the second PPDU.

According to a third aspect of the present invention, an exemplaryphysical protocol data unit (PPDU) transmission method is disclosed. Theexemplary PPDU transmission method includes: setting parameters of eachof a plurality of links for enabling the plurality of links to havedifferent capacity for PPDU transmission, wherein parameters of one ofthe plurality of links are different from parameters of another of theplurality of links, and highest capacity supported by said one of theplurality of links is higher than highest capacity supported by saidanother of the plurality of links; and transmitting, by a wirelessfidelity (WiFi) multi-link device (MLD), PPDUs via the plurality oflinks, wherein one PPDU is transmitted via said another of the pluralityof links, and multiple PPDUs are transmitted via said one of theplurality of links to ensure that an ending time instant of transmissionof said one PPDU and an ending time instant of transmission of themultiple PPDUs are aligned.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a wireless fidelity (WiFi) systemaccording to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a first physical protocol data unit(PPDU) ending time alignment mechanism according to an embodiment of thepresent invention.

FIG. 3 is a diagram illustrating a second PPDU ending time alignmentmechanism according to an embodiment of the present invention.

FIG. 4 is a diagram illustrating a third PPDU ending time alignmentmechanism according to an embodiment of the present invention.

FIG. 5 is a diagram illustrating a fourth PPDU ending time alignmentmechanism according to an embodiment of the present invention.

FIG. 6 is a diagram illustrating a fifth PPDU ending time alignmentmechanism according to an embodiment of the present invention.

FIG. 7 is a diagram illustrating a sixth PPDU ending time alignmentmechanism according to an embodiment of the present invention.

DETAILED DESCRIPTION

Certain terms are used throughout the following description and claims,which refer to particular components. As one skilled in the art willappreciate, electronic equipment manufacturers may refer to a componentby different names. This document does not intend to distinguish betweencomponents that differ in name but not in function. In the followingdescription and in the claims, the terms “include” and “comprise” areused in an open-ended fashion, and thus should be interpreted to mean“include, but not limited to . . . ”. Also, the term “couple” isintended to mean either an indirect or direct electrical connection.Accordingly, if one device is coupled to another device, that connectionmay be through a direct electrical connection, or through an indirectelectrical connection via other devices and connections.

FIG. 1 is a diagram illustrating a wireless fidelity (WiFi) systemaccording to an embodiment of the present invention. For brevity andsimplicity, it is assumed that the WiFi system 100 includes one accesspoint (AP) 102 and two non-AP stations (STAs) 104 and 106. In thisembodiment, AP 102 and non-AP STAs 104 and 106 are wireless fidelity(WiFi) multi-link devices (MLDs) that support multi-link operation(MLO). For example, AP 102 may own K links L₁-L_(K) at differentchannels (radio frequency bands), may communicate with non-AP STA 104via N links L₁₁-L_(1N) (which are selected from links L₁-L_(K)), and maycommunicate with non-AP STA 106 via M links L₂₁-L_(1M) (which areselected from links L₁-L_(K)), where K, M and N are positive integers, Nis not smaller than 2, M is not smaller than 2, and K is not smallerthan any of M and N. In some embodiments of the present invention,non-AP STAs 104 and 106 may be dual-radio STAs (M=N=2), and AP 102 maybe a tri-band AP (K=3). For example, links owned by AP 102 may include achannel in 5 GHz, a channel in 6 GHz, and a channel in 2.4 GHz. However,this is for illustrative purposes only, and is not meant to be alimitation of the present invention. Any WiFi system using the proposedphysical protocol data unit (PPDU) alignment mechanism falls within thescope of the present invention. In addition to MLO, the AP 102 maysupport other features such as multi-User multiple-input multiple-output(MU-MIMO) and orthogonal frequency-division multiple access (OFDMA) formulti-user transmission.

In certain cases, the PPDUs' starting time instants may not besynchronized due to the fact that transmission opportunity (TXOP) timingof multiple links may be different and/or the transmitter needs to checkclear channel assessment (CCA) for a certain period before mediumaccess. The proposed PPDU alignment mechanism is capable ofsynchronizing ending time instants of transmission of PPDUs transmittedover multiple links to help the acknowledgement (ACK) receiving and thefollow-up transmission to be synchronized. In this present invention,ending time instants of transmission of PPDUs transmitted over multiplelinks may be regarded as aligned with each other when mismatch betweenthe ending time instants falls within a predefined tolerance range. Forexample, the predefined tolerance range may be set by short interframespace (SIFS)±aSlotTime*10%. In practice, the predefined tolerance rangemay be adjusted, depending upon actual design considerations.

FIG. 2 is a diagram illustrating a first PPDU ending time alignmentmechanism according to an embodiment of the present invention. Inaccordance with the first PPDU ending time alignment mechanism, one WiFiMLD (e.g., AP 102 shown in FIG. 1 ) aligns parameters of a plurality oflinks (e.g., links L₁₁-L_(1N) or links L₂₁-L_(1M)) for constraining thelinks to have the same capacity for PPDU transmission, and transmitsPPDUs via the links, where highest capacity supported by one of thelinks is different from highest capacity supported by another of thelinks, and each of the PPDUs is generated and transmitted under the sameparameters. For example, the parameters may include BW, NSS, and MCS. Inthis embodiment, one WiFi MLD (e.g., AP 102) communicates with anotherWiFi MLD (e.g., Non-AP STA 104 or Non-AP STA 106) via a first link(denoted by “Link 1”) and a second link (denoted by “Link 2”), where thefirst link supports BW=40 MHz (denoted by “BW40”), NSS=1, and bestMCS=MCS3, and the second link supports BW=80 MHz (denoted by “BW80”),NSS=1, and best MCS=MCS7. Hence, the highest capacity supported by thesecond link is higher than the highest capacity supported by the firstlink.

In accordance with the first PPDU ending time alignment mechanism, thePPDU transmission for each of the first link and the second link may useBW=40 MHz, NSS=1, and best MCS=MCS3. The first PPDU ending timealignment mechanism may be applicable to the duplicate transmissionscenario with synchronized PPDU starting time. As shown in FIG. 2 , onePPDU 202 is transmitted via the first link, and one PPDU 204 istransmitted via the second link, where the WiFi MLD (e.g., AP 102)transmits the PPDUs 202 and 204 to the same user (e.g., non-AP STA 104or non-AP STA 106), and the PPDU 204 is a duplicate of the PPDU 202 suchthat the PPDUs 202 and 204 carry the same content. As shown in FIG. 2 ,the ending time instants of transmission of PPDUs 202 and 204 arealigned.

Since the highest capacity supported by the second link is higher thanthe highest capacity supported by the first link and parameters of thefirst link and the second link are constrained to be the same for PPDUtransmission, the available capacity of the second link is not fullyutilized by PPDU transmission. As a result, links capable of allowingmore data transmission may lose efficiency. As shown in FIG. 2 , a halfof the capacity supported by the second link is unused during atransmission period of the PPDU 204. To address this issue, the presentinvention proposes a second PPDU ending time alignment mechanism.

FIG. 3 is a diagram illustrating a second PPDU ending time alignmentmechanism according to an embodiment of the present invention. Inaccordance with the second PPDU ending time alignment mechanism, oneWiFi MLD (e.g., AP 102 shown in FIG. 1 ) aligns parameters of aplurality of links (e.g., links L₁₁-L_(1N) or links L₂₁-L_(2M) forconstraining the links to have the same capacity for PPDU transmission,and transmits PPDUs via the links, where highest capacity supported byone of the links is different from highest capacity supported by anotherof the links, each of the PPDUs is generated and transmitted under thesame parameters (which may include BW, NSS, and MCS), and more than onePPDU is transmitted via one link that supports higher capacity. Forexample, one WiFi MLD (e.g., AP 102) communicates with another WiFi MLD(e.g., non-AP STA 104 or non-AP STA 106) via a first link (denoted by“Link 1”) and a second link (denoted by “Link 2”), where the first linksupports BW=40 MHz (denoted by “BW40”), NSS=1, and best MCS=MCS3, andthe second link supports BW=80 MHz (denoted by “BW80”), NSS=1, and bestMCS=MCS7. Hence, the highest capacity supported by the second link ishigher than the highest capacity supported by the first link.

In accordance with the second PPDU ending time alignment mechanism, thePPDU transmission for each of the first link and the second link may useBW=40 MHz, NSS=1, and best MCS=MCS3. The second PPDU ending timealignment mechanism may be applicable to the duplicate transmissionscenario with synchronized PPDU starting time. As shown in FIG. 3 , onePPDU 302 is transmitted via the first link, and one PPDU 304 and oneduplicate PPDU 306 (which is a duplicate of PPDU 304) are transmittedvia the second link. In this embodiment, the WiFi MLD (e.g., AP 102)transmits the PPDUs 302, 304, and 306 to the same user (e.g., non-AP STA104 or non-AP STA 106), and the PPDU 304 is a duplicate of the PPDU 302such that the PPDUs 302, 304, and 306 carry the same content. As shownin FIG. 3 , the ending time instants of transmission of PPDUs 302, 304,and 306 are aligned. Duplicated transmission in extra BW of a highcapacity link (e.g., second link) further increases transmissionrobustness. In addition, extra signaling about the duplicate PPDU in theextra BW of the high capacity link (e.g., second link) may be required.

FIG. 4 is a diagram illustrating a third PPDU ending time alignmentmechanism according to an embodiment of the present invention. Inaccordance with the third PPDU ending time alignment mechanism, one WiFiMLD (e.g., AP 102 shown in FIG. 1 ) sets parameters of each of links forenabling the links to have different capacity for PPDU transmission,sets a content that is carried by a first PPDU transmitted via one ofthe links to ensure that an ending time instant of transmission of thefirst PPDU transmitted via one of the links and an ending time instantof transmission of a second PPDU transmitted via another of the linksare aligned, and transmits PPDUs (which include the first PPDU and thesecond PPDU) via the links. Parameters of each link may include BW, NSS,and MCS. In this embodiment, parameters of one of the links aredifferent from parameters of another of the links, highest capacitysupported by one of the links is higher than highest capacity supportedby another of the links, and one PPDU is transmitted via each of thelinks. Each link may use its best parameters to generate and transmitthe PPDU. For example, one WiFi MLD (e.g., AP 102) communicates withanother WiFi MLD (e.g., non-AP STA 104 or non-AP STA 106 shown in FIG. 1) via a first link (denoted by “Link 1”) and a second link (denoted by“Link 2”), where the first link supports BW=40 MHz (denoted by “BW40”),NSS=1, and best MCS=MCS3, and the second link supports BW=80 MHz(denoted by “BW80”), NSS=1, and best MCS=MCS7. Hence, the highestcapacity supported by the second link is higher than the highestcapacity supported by the first link. The first link is configured touse best parameters, including BW=40 MHz, NSS=1, and best MCS=MCS3, forPPDU transmission, and the second link is configured to use bestparameters, including BW=80 MHz, NSS=1, and best MCS=MCS7, for PPDUtransmission.

Since the capacity provided by the second link is higher than thecapacity provided by the first link, partial duplication can be enabledto align PPDU lengths for achieving PPDU ending time alignment. Thethird PPDU ending time alignment mechanism may be applicable to all ofthe aforementioned scenarios. As shown in FIG. 4 , one PPDU 402 istransmitted via the first link, and one PPDU 404 is transmitted via thesecond link, where the WiFi MLD (e.g., AP 102) transmits PPDUs 402 and404 to the same user (e.g., non-AP STA 104 or non-AP STA 106), and thePPDU 404 transmitted via the second link includes a partial PPDU 406(which includes one or more media access control protocol data units(MPDUs) 407), one or more duplicate MPDUs 408, and optional padding bits410. The duplicate MPDU(s) 408 may be derived from duplication ofMPDU(s) 407, and one or more duplicate MPDUs may be obtained from thesame source MPDU. The optional padding bits 410 may be set by 0's. Inthis embodiment, the duplicate MPDU(s) 408 are appended to the partialPPDU 406, and the optional padding bits 410 are appended to theduplicate MPDU(s) 408. In an alternative design, source MPDU(s) 407 andduplicate MPDU(s) 408 may be interleaved in the PPDU 404 to increasediversity. As shown in FIG. 4 , the ending time instants of transmissionof PPDUs 402 and 404 are aligned. Signaling of MPDU duplication may berequired, and MPDUs with the same content can be decoded individually orjointly to reduce error possibility. It should be noted that PPDUstarting time alignment is not necessarily required by the third PPDUending time alignment mechanism.

FIG. 5 is a diagram illustrating a fourth PPDU ending time alignmentmechanism according to an embodiment of the present invention. Inaccordance with the fourth PPDU ending time alignment mechanism, oneWiFi MLD (e.g., AP 102 shown in FIG. 1 ) sets parameters of links forenabling the links to have different capacity for PPDU transmission, andtransmits PPDUs via the links. Parameters of each link may include BW,NSS, and MCS. In this embodiment, parameters of one of the links aredifferent from parameters of another of the links, highest capacitysupported by one of the links is higher than highest capacity supportedby another of the links, one PPDU is transmitted via one link, andmultiple PPDUs are transmitted via another link to ensure that an endingtime instant of transmission of one PPDU on one link and an ending timeinstant of transmission of multiple PPDUs on another link are aligned.

Each link may use its best parameters to generate and transmit the PPDU.For example, one WiFi MLD (e.g., AP 102) communicates with another WiFiMLD (e.g., non-AP STA 104) via multiple links including a first link(denoted by “Link 1”) and a second link (denoted by “Link 2”), andfurther communicates with yet another WiFi MLD (e.g., non-AP STA 106)via multiple links including the second link (denoted by “Link 2”),where the first link supports BW=40 MHz (denoted by “BW40”), NSS=1, andbest MCS=MCS3, and the second link supports BW=80 MHz (denoted by“BW80”), NSS=1, and best MCS=MCS7. Hence, the highest capacity supportedby the second link is higher than the highest capacity supported by thefirst link. The first link is configured to use best parameters,including BW=40 MHz, NSS=1, and best MCS=MCS3, for PPDU transmission,and the second link is configured to use best parameters, includingBW=80 MHz, NSS=1, and best MCS=MCS7, for PPDU transmission.

Since the capacity provided by the second link is higher than thecapacity provided by the first link, the second link is allowed totransmit multiple PPDUs during a transmission period of one PPDU that istransmitted via the first link. The fourth PPDU ending time alignmentmechanism may be applicable to all of the aforementioned scenarios. Asshown in FIG. 5 , one PPDU 502 is transmitted via the first link, andone PPDU 504 and at least one padding PPDU 506 are transmitted via thesecond link, where the WiFi MLD (e.g., AP 102) transmits the PPDUs 502and 504 to the same user (e.g., non-AP STA 104), and transmits paddingPPDU(s) 506 to a different user (e.g., non-AP STA 106). The multiplePPDUs 504 and 506 transmitted via the second link are separated by SIFSto ensure medium occupation. As shown in FIG. 5 , an ending time instantof transmission of PPDU 502 and an ending time instant of transmissionof multiple PPDUs 504 and 506 (particularly, an ending time instant oftransmission of the last one of PPDUs 504 and 506) are aligned. Itshould be noted that PPDU starting time alignment is not necessarilyrequired by the fourth PPDU ending time alignment mechanism.

In a case where the fourth PPDU ending time alignment mechanism isapplied to the duplicate transmission scenario, the PPDU 504 is aduplicate of the PPDU 502, and an ending time instant of transmission ofthe PPDU 504 is earlier than an ending time instant of transmission ofthe PPDU 502. If immediate acknowledgement (ACK) is required byduplicate transmission and an ACK message is sent over the second link,the ACK message receiving interferes with transmission of paddingPPDU(s) 506. To address this issue, the present invention proposessending the ACK message over the first link that occupies the longerperiod to avoid interfering with transmission of the padding PPDU(s) 506on the second link. Alternatively, a block ACK mechanism may be adoptedto collect acknowledgement later after the transmission, and/orsignaling can be adopted to delay the acknowledgement until the end oftransmission.

FIG. 6 is a diagram illustrating a fifth PPDU ending time alignmentmechanism according to an embodiment of the present invention. Inaccordance with the fifth PPDU ending time alignment mechanism, one WiFiMLD (e.g., AP 102 shown in FIG. 1 ) sets parameters of links forenabling the links to have different capacity for PPDU transmission,sets a content that is carried by a first PPDU transmitted via one ofthe links to ensure that an ending time instant of transmission of thefirst PPDU transmitted via one of the links and an ending time instantof transmission of a second PPDU transmitted via another of the linksare aligned, and transmits PPDUs (which include the first PPDU and thesecond PPDU) via the links. Parameters of each link may include BW, NSS,and MCS. In this embodiment, parameters of one of the links aredifferent from parameters of another of the links, highest capacitysupported by one of the links is higher than highest capacity supportedby another of the links, and one PPDU is transmitted via each of thelinks. Each link may use its best parameters to generate and transmitPPDU. For example, one WiFi MLD (e.g., AP 102) communicates with anotherWiFi MLD (e.g., non-AP STA 104 or non-AP STA 106) via a first link(denoted by “Link 1”) and a second link (denoted by “Link 2”), where thefirst link supports BW=40 MHz (denoted by “BW40”), NSS=1, and bestMCS=MCS3, and the second link supports BW=80 MHz (denoted by “BW80”),NSS=1, and best MCS=MCS7. Hence, the highest capacity supported by thesecond link is higher than the highest capacity supported by the firstlink. The first link is configured to use best parameters, includingBW=40 MHz, NSS=1, and best MCS=MCS3, for PPDU transmission, and thesecond link is configured to use best parameters, including BW=80 MHz,NSS=1, and best MCS=MCS7, for PPDU transmission.

Since the capacity provided by the second link is higher than thecapacity provided by the first link, different MPDUs can be allowed inthe second link to align the PPDU lengths for achieving PPDU ending timealignment. The fifth PPDU ending time alignment mechanism may beapplicable to the duplication transmission scenario with no PPDUstarting time alignment. As shown in FIG. 6 , one PPDU 602 istransmitted via the first link, and one PPDU 604 is transmitted via thesecond link, where the WiFi MLD (e.g., AP 102) transmits PPDUs 602 and604 to the same user (e.g., non-AP STA 104 or non-AP STA 106), and thePPDU 604 transmitted via the second link includes a partial PPDU 606(which includes one or more media access control protocol data units(MPDUs) 607), and one or more other MPDUs 608 that are not duplicate(s)of MPDU(s) 607. The MPDU(s) 608 are appended to the partial PPDU 606. Asshown in FIG. 6 , an ending time instant of transmission of PPDU 602 andan ending time instant of transmission of PPDU 604 are aligned. Thepartial PPDU 606 may be a duplicate of the PPDU 602. Hence, the MPDU(s)608 carried in the second link are different from the MPDU(s) carried inthe first link. In some embodiments of the present invention, the PPDU604 may include optional padding bits (not shown) that are appended tothe MPDU(s) 608 to make the PPDU length aligned.

FIG. 7 is a diagram illustrating a sixth PPDU ending time alignmentmechanism according to an embodiment of the present invention. Inaccordance with the sixth PPDU ending time alignment mechanism, one WiFiMLD (e.g., AP 102 shown in FIG. 1 ) sets parameters of links forenabling the links to have different capacity for PPDU transmission,sets a content that is carried by a first PPDU transmitted via one ofthe links to ensure that an ending time instant of transmission of thefirst PPDU transmitted via one of the links and an ending time instantof transmission of a second PPDU transmitted via another of the linksare aligned, and transmits PPDUs (which include the first PPDU and thesecond PPDU) via the links. Parameters of each link may include BW, NSS,and MCS. In this embodiment, parameters of one of the links aredifferent from parameters of another of the links, highest capacitysupported by one of the links is higher than highest capacity supportedby another of the links, and one PPDU is transmitted via each of thelinks. Each link may use its best parameters to generate and transmitthe PPDU. For example, one WiFi MLD (e.g., AP 102) communicates withanother WiFi MLD (e.g., non-AP STA 104) via multiple links including afirst link (denoted by “Link 1”) and a second link (denoted by “Link2”), and communicates with yet another WiFi MLD (e.g., non-AP STA 106)via multiple links including the second link (denoted by “Link 2”),where the first link supports BW=40 MHz (denoted by “BW40”), NSS=1, andbest MCS=MCS3, and the second link supports BW=80 MHz (denoted by“BW80”), NSS=1, and best MCS=MCS7. Hence, the highest capacity supportedby the second link is higher than the highest capacity supported by thefirst link. The first link is configured to use best parameters,including BW=40 MHz, NSS=1, and best MCS=MCS3, for PPDU transmission,and the second link is configured to use best parameters, includingBW=80 MHz, NSS=1, and best MCS=MCS7, for PPDU transmission.

Since the capacity provided by the second link is higher than thecapacity provided by the first link, the second link can transmit amulti-user (MU) PPDU under a condition that the PPDU ending timealignment is met. The sixth PPDU ending time alignment mechanism may beapplicable to all of the aforementioned scenarios. As shown in FIG. 7 ,one PPDU 702 is transmitted via the first link, and one MU PPDU 704 istransmitted via the second link. The MU PPDU 704 accommodates data ofdifferent users (e.g., different WiFi MLDs) in a plurality of resourceunits (RUs), where each RU is a group of subcarriers (tones). Forexample, the MU PPDU 704 carries user data D_RU1 in a first RU of the MUPPDU 704 and user data D_RU2 in a second RU of the MU PPDU 704, wherethe user data D_RU1 is transmitted to one user (e.g., non-AP STA 104),and the user data D_RU2 is transmitted to another user (e.g., non-AP STA106). As shown in FIG. 7 , an ending time instant of transmission ofPPDU 702 and an ending time instant of transmission of MU PPDU 704 arealigned. In some embodiments of the present invention, the PPDU 702 andthe user data D_RU1 included in the MU PPDU 704 may be transmitted tothe same user (e.g., non-AP STA 104 or non-AP STA 106). In addition,padding in RUs for multiple users may be required. It should be notedthat PPDU starting time alignment is not necessarily required by thesixth PPDU ending time alignment mechanism.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. A physical protocol data unit (PPDU) transmissionmethod comprising: aligning, by a wireless fidelity (WiFi) multi-linkdevice (MLD), parameters of a plurality of links for constraining theplurality of links to have same capacity for PPDU transmission, whereinthe parameters comprise bandwidth (BW), number of spatial streams (NSS),and modulation and coding mechanism (MCS), the plurality of links use asame setting of BW, a same setting of NSS, and a same setting of MCS,and highest capacity supported by one of the plurality of links isdifferent from highest capacity supported by another of the plurality oflinks; and transmitting PPDUs via the plurality of links, wherein eachof the PPDUs is generated and transmitted under the same parameters, oneof the PPDUs is transmitted via said one of the plurality of links,another of the PPDUs is transmitted via said another of the plurality oflinks, and an ending time instant of transmission of said one of thePPDUs is aligned with an ending time instant of transmission of saidanother of the PPDUs.
 2. The PPDU transmission method of claim 1,wherein the highest capacity supported by said one of the plurality oflinks is higher than the highest capacity supported by said another ofthe plurality of links, the highest capacity supported by said anotherof the plurality of links is available under first parameters employedby said another of the plurality of links for PPDU transmission, andaligning the parameters of the plurality of links adopted by the WiFiMLD comprises: setting second parameters employed by said one of theplurality of links for PPDU transmission by the first parameters.
 3. ThePPDU transmission method of claim 2, wherein transmitting the PPDUs viathe plurality of links comprises: transmitting one PPDU via said one ofthe plurality of links, wherein said one of the plurality of links hasunused capacity during a transmission period of said one PPDU.
 4. ThePPDU transmission method of claim 2, wherein transmitting the PPDUs viathe plurality of links comprises: transmitting one PPDU and oneduplicate PPDU via said one of the plurality of links, wherein said onePPDU and said one duplicate PPDU carry a same content.
 5. The PPDUtransmission method of claim 1, wherein transmitting the PPDUs via theplurality of links comprises: transmitting a first PPDU via said one ofthe plurality of links; and transmitting a second PPDU via said anotherof the plurality of links; wherein the first PPDU and the second PPDUcarry a same content.
 6. The PPDU transmission method of claim 1,wherein the PPDUs are transmitted to a same user only.
 7. A wirelessfidelity (WiFi) multi-link device (MLD), arranged to perform operationscomprising: aligning parameters of a plurality of links for constrainingthe plurality of links to have same capacity for physical protocol dataunit (PPDU) transmission, wherein the parameters comprise bandwidth(BW), number of spatial streams (NSS), and modulation and codingmechanism (MCS), the plurality of links use a same setting of BW, a samesetting of NSS, and a same setting of MCS, and highest capacitysupported by one of the plurality of links is different from highestcapacity supported by another of the plurality of links; andtransmitting PPDUs via the plurality of links, wherein each of the PPDUsis generated and transmitted under the same parameters, one of the PPDUsis transmitted via said one of the plurality of links, another of thePPDUs is transmitted via said another of the plurality of links, and anending time instant of transmission of said one of the PPDUs is alignedwith an ending time instant of transmission of said another of thePPDUs.
 8. The WiFi MLD of claim 7, wherein the highest capacitysupported by said one of the plurality of links is higher than thehighest capacity supported by said another of the plurality of links,the highest capacity supported by said another of the plurality of linksis available under first parameters employed by said another of theplurality of links for PPDU transmission, and aligning the parameters ofthe plurality of links adopted by the WiFi MLD comprises: setting secondparameters employed by said one of the plurality of links for PPDUtransmission by the first parameters.
 9. The WiFi MLD of claim 8,wherein transmitting the PPDUs via the plurality of links comprises:transmitting one PPDU via said one of the plurality of links, whereinsaid one of the plurality of links has unused capacity during atransmission period of said one PPDU.
 10. The WiFi MLD of claim 8,wherein transmitting the PPDUs via the plurality of links comprises:transmitting one PPDU and one duplicate PPDU via said one of theplurality of links, wherein said one PPDU and said one duplicate PPDUcarry a same content.
 11. The WiFi MLD of claim 7, wherein transmittingthe PPDUs via the plurality of links comprises: transmitting a firstPPDU via said one of the plurality of links; and transmitting a secondPPDU via said another of the plurality of links; wherein the first PPDUand the second PPDU carry a same content.
 12. The WiFi MLD of claim 7,wherein the PPDUs are transmitted to a same user only.